Field of the Invention
The present invention relates to an image forming apparatus that controls image density.
Description of the Related Art
A full color image forming apparatus that employs an electrophotographic system forms toner images of color components by developing electrostatic latent images formed on photosensitive members with developers including toners of the-color components. The toner images of the color components are transferred to an intermediate transfer medium in piles by a transfer unit, so that a full color toner image corresponding to an original appears. The toner image transferred to the intermediate transfer medium is again transferred to a recording material like a sheet, and then is fixed to the recording material by applying heat and pressure by a fixing unit. The recording material to which the toner image has been fixed is output from the image forming apparatus as a printed matter.
Such an image forming apparatus controls image forming conditions, such as an exposure light amount for forming an electrostatic latent image on a photosensitive member, exposure time, developing bias, and electrification potential, in order to control density of an output image. However, even if the image formation conditions are controlled so as to be constant, image density may vary because of temporal changes of states of the image forming apparatus, such as a charge amount of toner, sensitivity of a photosensitive member, and a transfer efficiency, and changes of environmental conditions, such as temperature and humidity.
For example, in a system that develops a latent image by a predetermined development contrast potential, deviation of a charge amount of toner from a reference value changes a toner amount (the number of toner particles) required to satisfy the development contrast potential. That is, an increase of the charge amount of toner decreases the toner amount, and a decrease of the charge amount of toner increases the toner amount. Then, the decrease of the toner amount used for development lowers density of a toner image. Moreover, the change of the sensitivity of the photosensitive member due to the temporal changes or the changes of the environmental conditions may also change the electric potential on the photosensitive member.
The image density varies in response to the changes of the image forming conditions or of the states of the image forming apparatus.
There is a conventional technique that forms a test pattern on an image bearing member, such as a photosensitive member and an intermediate transfer belt. The test pattern is detected with an optical sensor, and image forming conditions, such as an exposure light amount and exposure time, are feedback-controlled on the basis of a detection result.
However, it is known that a huge amount of image formations change a glossiness of a surface of an image bearing member over time in an image forming apparatus. Since a reflectance of the image bearing member varies according to the temporal variation of the glossiness of the image bearing member, a reflected light amount varies over time even if a toner amount of a toner image on the image bearing member is constant.
There is a proposed technique that finds a correction amount on the basis of a reflected light amount from an image bearing member on which a test pattern will be formed and corrects a reflected light amount from the test pattern using the correction amount (for example, see US 2002/0110381). The image forming apparatus disclosed in this publication predicts a correction coefficient corresponding to the change of the reflectance of the image bearing member, and corrects the detection result of the test pattern on the basis of the correction coefficient.
However, a sensor output is affected by ratio (no-toner area ratio) of an area where the toner of the test pattern does not cover the image bearing member in a sensor detection area. Accordingly, the image forming apparatus needs to correct the detection result of the sensor in consideration of the no-toner area ratio and the change of the reflectance of the image bearing member. Particularly, when a low-density test pattern with a high no-toner area ratio is detected, density of an output image cannot be corrected with high accuracy unless both the no-toner area ratio and the change of the reflectance of the image bearing member are considered.